1. Field of the Invention
Embodiments of the invention generally relate to matching networks for semiconductor processing systems. More specifically, embodiments of the present invention relate to characterization of matching networks.
2. Description of the Related Art
In plasma-enhanced semiconductor processing systems, such as etch or deposition processing systems, radio frequency (RF) matching networks are used to couple RF power from an RF source having a substantially resistive impedance (e.g., 50 ohms) to a load, which is generally a processing plasma in a process chamber, which has a complex impedance. The matching network operates to match the RF source's impedance to the plasma/load's impedance to efficiently couple the RF power from the source to the plasma.
One type of matching network that is widely used in semiconductor wafer processing systems is a tunable matching network, wherein a series connected frequency-dependent passive element and a shunt connected frequency-dependent passive element are dynamically tuned to achieve an impedance match between the source and the load. One such tunable matching network is disclosed in commonly assigned U.S. Pat. No. 5,952,896, which issued on Sep. 14, 1999 and describes a matching network having a series connected inductor and a shunt connected capacitor. A matching network controller mechanically tunes or adjusts the shunt position of the capacitor and the inductor to achieve an impedance match between the source impedance and the load impedance. More particularly, as the load impedance changes, actuators continually adjust the tunable elements of the inductor and capacitor in the matching network to maintain the impedance match.
In a tunable matching network, the matching network terminates into a non-dissipating load (generally a capacitive element), which allows the matching network to tune and measure current. In this situation, when the power is known and current is accurate, RMW=Pin/I2. However, drawbacks to this computational method include: a) only one match set point can be tested; b) the matching network typically cannot tune to a zero resistance load condition, which results in a mismatch condition that reduces the accuracy of the power measurement (and therefore increases the inaccuracy of the resistance calculation); and c) the matching network requires an accurate measurement of RF current at the match output for proper operation, which is not provided by conventional configurations.
Additionally, conventional matching networks typically introduce purely reactive elements in series with a 50Ω terminated load to move the total impedance of the load into the matching network's operative tuning range. In this configuration, all of the power is dissipated in the 50Ω load, which allows for a more accurate measurement at that particular point. If the matching network has a resistance that is also very close to 50Ω (or close to the terminated load resistance), the power loss through the matching network can be accurately measured using two power meters. If the impedance of the termination load is known, this can be converted into an equivalent series resistance for the match. However, building a complex, purely reactive load is very difficult. Therefore, unaccounted for losses in the surrogate load will overestimate the power dissipation in the matching network, resulting in higher than actual resistance approximations for conventional matching networks of this configuration.
Therefore, there is a need for a matching network analysis technique that allows for improved approximation of an equivalent series resistance of matching networks.